G. Carl Huber . 25 



other parts of the vesicles and that evidences of cell division are often 

 seen. 



In such a section it will be seen that the lower inner ends of these 

 elongated cells, which no longer hold a radial position to the lumen of. 

 the vesicle, but are inclined to its long axis, slighth^ overlap the inner 

 ends of the cells which form the lower portion of the outer wall of the 

 vesicle, so that a portion of the outer wall of the vesicle presents the 

 appearance of a two-layered epithelium. As development proceeds, the 

 outer wall of the vesicle further thickens and encroaches on its lumen, 

 which in sagittal sections now presents the appearance of a hook-shaped 

 space. At about this time there appears on the outer wall of the vesicle 

 over its thickest portion a slight depression, beneath wdiich a cleft makes 

 its appearance; this begins to separate the thickest portion of the wall 

 of the vesicle into two layers. This cleft deepens as the two layers of 

 cells become more distinctly separated. In A of Fig. 3 is shown a sagit- 

 tal section of a renal vesicle at this stage of development. The section 

 sketched does not pass exactly through the center of the vesicle, nor is it 

 quite parallel to its long axis. This needs to be remembered in exam- 

 ining this figure, as the arrangement and size of the cells forming the 

 different parts of the wall of the vesicle are not qiiite what might be 

 expected from the foregoing description. The reproduction gives the 

 most typic section of the series of sagittal sections of the vesicle from 

 which the reconstruction was made which is shown in A of Pig. 4. In 

 A of Fig. 3 is shown an ampullar enlargement of a terminal branch of a 

 collecting tubule surrounded by nephrogenic tissue, a renal vesicle 

 and the surrounding mesenchyme and a portion of the capsule. 

 In the wall of the vesicle away from the collecting tubule may be 

 observed a slight depression and beneath this a, cleft. The nuclei of 

 the cells adjacent to the cleft are stained a little more deeply than the 

 other nuclei of the vesicle. Such an appearance is now and then seen, 

 though not characteristic of this stage. In the reconstruction, this cleft 

 appears on the surface as a narrow depression placed nearly transversely 

 to the long axis of the vesicle, a little above its center, though not ex- 

 tending across its entire wall, and is therefore scarcely seen in the ex- 

 posure of the model as sketched. That portion of the vesicle, which in 

 sagittal section shows the narrow cleft, is in reality a small pocket, the 

 upper and lower walls of which are nearly in contact at this stage of the 

 development. Similar appearances I have observed in a number of 

 other wax reconstructions of this stage. This pocket is not developed 

 primarily by an infolding or invagination of the outer wall of the vesicle, 

 but differentiates in a thickening of its outer wall. This thickening 



